Engine/generator driven welding power supplies typically include an engine, a generator, a welding power source, often a wire feeder, and one or more controllers. The components may be sold as a group, a single unit or individually.
The engine has a throttle which may have a range of speed selections and often has a control mechanism to allow operation at full throttle and idle. The generator output is often controlled using field current, and the welding power supply is controlled in response to a user set point or reference. The controller may be relatively complex, and control the components in a unified manner, or it may be individual controllers for each component, with little or no joint control of components.
Many controllers that treat the components as individual components rather than as part of a system may be inadequate. For example, a typical engine control (used outside of the welding art) might not be capable of responding to the very rapid increase in power needed when an arc is struck or the set point changed. Similarly, a welding power supply controller designed for utility power line voltage use might not adequately respond when the engine is slow to provide the needed power.
A controller for field current designed for a welding power supply is described in U.S. Pat. No. 5,734,147, entitled Method And Apparatus For Electronically Controlling The Output Of A Generator Driven Welding Power Supply, filed Sep. 8, 1995, and assigned to the assignee of the present invention. U.S. Pat. No. 5,734,147, is hereby incorporated by reference.
One problem with engine/generator driven welding power supplies that are not properly controlled is that it may be difficult to start the arc, especially in stick or MIG welding. Generally, a "hot" start or higher current/power start is desirable so the stick does not become welded to the workpiece. But before an arc is initiated or struck, an engine/generator driven welding power supply generator is idling. It cannot provide a hot or high current/power start because the horsepower (which is transformed into output power) available while idling is much less than the horsepower available at higher RPM.
Experienced welders have attempted to get a hot start by "double striking" or touching the stick to the workpiece before welding. This causes current to increase, and the engine to speed up out of idle, before the arc is struck. However, this caused marking of the workpiece that was unacceptable for some x-ray quality welds or welds requiring a high surface quality. Also, this may not be a satisfactory solution for heavy loads or for CV applications.
Accordingly, U.S. Pat. No. 5,734,147 teaches to provide a hot start by giving an additional boost of current when the arc is struck. Unfortunately, this sometimes caused the engine to stall, because the called for output power was greater than the horsepower available (after accounting for system inefficiencies which may be about 50%) at lower RPM.
An engine/generator driven welding power supply is likely to stall when more power is drawn than is capable of being provided. This is more likely to happen at lower RPM, because less horsepower is available at lower RPM than at higher RPM.
One attempted solution to that problem is to increase the engine speed at idle. This undesirably increases fuel consumption and reduces engine life. An alternative attempted solution is to increase the engine speed from idle to a higher speed. Another attempted solution is to temporarily reduce the output, until the likelihood of a stall is reduced. Both of these methods are suggested in application Ser. No. 08/858,129, filed May 19, 1997, entitled Engine Driven Invertor With Feedback Control, which is owned by the assignee of this invention and is hereby incorporated by reference.
However, the response of engine speed to throttle changes is often not fast enough to prevent stalling, particularly if the load (output power) had been quickly increased when the engine is at a low speed. Also, reducing output power can cause additional problems--lower power can have an adverse impact on the arc. Thus, the competing interests of reducing power to avoid engine stalls while maintaining power to maintain a quality arc were necessarily properly balanced, particularly when the power reduction was merely on or off, and not variably controlled.
For example, the prior art does not teach to reduce the output by an amount responsive to operating conditions such as engine speed or output current, power, load, setpoint etc. In other words, it does not teach to have greater reduction in output power when the engine is slower, or the output is greater--the reduction is the same regardless of the severity of the stall conditions and the conditions of the arc.
The Miller.TM. BlueStar.TM. welding power supply tried to account for different severity of stall conditions by increasing the "throttle back" of output power as RPM decreased. This was done for all output currents even though at some output currents the load was not large enough to be likely to cause a stall. The reduction at lower currents often unduly reduced the quality of the arc. Thus, this system did not balance the need for power reduction to avoid a stall, and the need for power maintenance to help arc quality.
Accordingly, a controller for an engine/generator driven welder that reduces the output in response to potential stall conditions, and that does so at a magnitude responsive to the severity of the stall conditions and the arc condition is desirable.